#include<bits/stdc++.h>
//#pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math")
//#pragma GCC optimize(2)
//#pragma GCC optimize(3)
//#pragma GCC optimize("Ofast,unroll-loops")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4.1,sse4.2,avx,avx2,popcnt,tune=native")
//#include <immintrin.h>
//#include <emmintrin.h>
//#define int long long
#define ls(x) ((x)*2)
#define rs(x) ((x)*2+1)
#define pii pair<int,int>
#define fi first
#define se second
#define Debug(...) fprintf(stderr, __VA_ARGS__)
#define For(i,a,b) for(int i=a,i##end=b;i<=i##end;i++)
#define Rof(i,a,b) for(int i=a,i##end=b;i>=i##end;i--)
#define rep(i,  b) for(int i=1,i##end=b;i<=i##end;i++)
using namespace std;
const int N=1e6+5,base=999983,Mod=998244353;
char buf[(1<<21)+5],*p1,*p2;
#define getchar() (p1==p2&&(p2=(p1=buf)+fread(buf,1,1<<21,stdin),p1==p2)?EOF:*p1++)
inline void chmx(int &x,int y){(x<y)&&(x=y);}
inline void chmn(int &x,int y){(x>y)&&(x=y);}
inline void Add(int &x,int y){(x=x+y+Mod)%=Mod;}
inline int read(){
	int f=0,x=0;
	char ch=getchar();
	while(!isdigit(ch)){f|=(ch=='-');ch=getchar();}
	while(isdigit(ch)){x=(x<<3)+(x<<1)+(ch^48);ch=getchar();}
	return f?-x:x;
}
void print(int n){
    if(n<0){
        putchar('-');
        n*=-1;
    }
    if(n>9) print(n/10);
    putchar(n%10+'0');
}
int n,k;
pii a[N];
int gg[N],tot;
double suf[N]; 
double A[N],B[N];
inline double work(){
	double ss=1;
	Rof(i,k,1) ss=ss*B[gg[i]];
	double p=1,res=0;
	For(t,0,k){
		ss=ss*A[gg[t]];
		res+=ss;
	}
	return res;
}double answer=0;
double mp[N];
bool vis[N];
inline double check(int x){
	if(x==k+1) return 0;
	if(vis[x])return mp[x];
	tot=0;
	For(i,1,x)gg[++tot]=i;
	For(i,n-(k-x)+1,n)gg[++tot]=i;
	double res=work();
	answer=max(answer,res);
	vis[x]=1;mp[x]=res;
	return res;
}
signed main(){
	freopen("sleep.in","r",stdin);
	freopen("sleep.out","w",stdout);
	// ios::sync_with_stdio(false);
	// cin.tie(0); cout.tie(0);
	n=read(),k=read();
	For(i,1,n){
		a[i].fi=read(),a[i].se=read();
	}
	sort(a+1,a+n+1,[](pii x,pii y){return 1.0*x.fi*y.se>1.0*x.se*y.fi;}); 
	A[0]=1;
	suf[k+1]=1.0;B[k+1]=1.0;
	For(i,1,n) A[i]=1.0*a[i].fi/(a[i].se-a[i].fi);
	For(i,1,n) B[i]=1.0*(a[i].se-a[i].fi)/a[i].se;
	if(n<=20){
		double ans=0;int p=0;
		For(i,0,(1<<n)-1){
			if(__builtin_popcount(i)!=k) continue;
			tot=0;
			For(j,1,n){
				if(i&(1<<(j-1)))gg[++tot]=j;
			}
			ans=max(ans,work());
		}
		cout<<fixed<<setprecision(9)<<1.0-ans<<endl;
		return 0;
	}
	int l=0,r=k;
	while(l<=r){
		int mid=(l+r)>>1;
		if(check(mid)<=check(mid+1))l=mid+1;
		else r=mid-1;
	}cout<<fixed<<setprecision(9)<<1.0-answer<<endl;
#ifdef LOCAL
    Debug("\nMy Time: %.3lfms\n", (double)clock() / CLOCKS_PER_SEC);
#endif
	return 0;
}

